Conventionally, a technology in which a multi-link, piston-crank mechanism is used for connecting a piston pin and a crank pin of an internal combustion engine is known such as in Japanese Patent Application Publication No. 2010-185329. This is provided with an upper link connected to a piston pin of a piston, a lower link connecting this upper link and a crank pin of a crankshaft, and a control link with its one end supported pivotally on the side of an engine body and with the other end connected to the lower link. The upper link and the lower link are connected via an upper pin rotatably relative to each other while the control link and the lower link are connected via a control pin for being rotatable relative to each other.
The lower link in the piston-crank mechanism of the multi-link type is subjected to receive a combustion pressure (cylinder pressure) through the upper link, and the force is transferred to the crank pin by way of a sort of “lever” with fulcrum positioned on the control pin. Accordingly, the lower link is exposed to a high combustion pressure or inertial load received by a piston from a bearing portion of the upper pin through the piston pin, the upper link and the upper pin. At the same time, so as to balance with the load, the load is also generated in the control pin bearing portion and the crank pin bearing portion. Therefore, the surface pressure of each bearing portion will be severe as compared to a conventional reciprocating engine of the single-link in order to prevent seizing and wear and to maintain sufficient lubrication state.
In particular, with respect to the lubrication of the bearing portion of the upper pin connecting the lower link and the upper link, while the lower link revolves around the crankshaft with the crank pin in response to rotation of the crankshaft during engine operation, it also displaces in the rotational direction with respect to this crank pin. Therefore, it is difficult to supply lubricating oil forcibly by using the hydraulic pressure.
Thus, in the technique described above, an oil passage is formed in each of the crank pin, the lower link, and the upper pin in their pin boss portions, respectively so as to establish communication of the oil between the oil passage of the crank pin and that of the lower link. Thus lubricating oil will be injected from the oil passage in the lower link toward the oil passage of the pin boss portion so as to provide lubricating oil to the bearing portion of the pin boss portion of the upper link via this oil passage of the lower link.